A spool valve comprises a spool slidably mounted within a valve body. In operation the spool is driven between different positions within the valve body. The spool serves to connect one or more inlets with one or more outlets of the valve. The position of the spool within the valve body affects the connection(s) between inlets and outlets. For example the spool may have one or more lands that separate grooves on the spool. The lands seal against the valve body and thus prevent or restrict fluid flow past the lands. An inlet and an outlet are connected together when they both align with a single groove of the spool. Movement of the spool may cause a land to move past an inlet or outlet, thus changing the connection by moving the connecting groove out of alignment with at least one of the inlet and outlet. A land may also partially obstruct an inlet or outlet, thus restricting the fluid connection (i.e. an intermediate position between fully open and fully closed).
The spool positions for certain valve states may be defined by the limit of movement of the spool within the valve body, i.e. where the spool is driven as far as possible to one side or the other side of the valve body. These limits of motion are defined by bumpers or plugs mounted within the valve body.
For proper operation of a spool valve, the limits of motion of the spool within the valve body must be accurately defined so that the various inlets and outlets of the valve are appropriately connected in the different valve states. For example if the movement of the spool is restricted too much, it may partially block an inlet or outlet, restricting the flow through the valve. Equally, if the movement of the spool is not restricted enough, the spool may overshoot and partially block an inlet or outlet, again restricting the flow through the valve.
Assembling a spool valve involves inserting a spool into a valve body and inserting two plugs or stoppers (also referred to as bumpers), one at each end of the valve body that define the limits of the spool movement within the valve body. The valve is then calibrated by adjusting the size (i.e. length) of the plugs or stoppers so as to define the limits of spool movement.
A commonly used method of spool stroke calibration of a servo valve involves machining or grinding of the bumper to the desired length. This involves measurement of the actual parts' geometry and correction of their dimensions. This may involve replacement of the stopper with one of a different length or grinding of the stopper to remove material, shortening it to a desired length. In the case of hydraulic calibration, the method involves assembling the spool and bumper(s) in the valve body, performing an examination or test of the valve and, if necessary, removing the bumper(s) together with fasteners and seals and replacing it (or them), or alternatively grinding it (or them) to size. The valve is then reassembled and the stroke test repeated. This process is complicated and significantly increases the time required for assembly of the valve.